http://primecoin.io/ http://primecoin.io/ is an example where the work being done is useful - it is used to discover more prime numbers. However, regardless of

http://primecoin.io/ http://primecoin.io/ is an example where the work being done is useful - it is used to discover more prime numbers. However, regardless of the "usefulness" of the actual work, it ties the value of bitcoin to a very tangible resource: the amount of mining power available in the network. This is actually more similar to the gold standard than fiat currency, where proven reserves are needed to print more currency.